Optical Sky Background

The optical sky background
depends on a number of parameters including the target - moon angular
separation, lunar phase, ecliptic latitude, zenith angle, and phase of the
solar cycle (e.g. Krisciunas 1997, PASP, 209, 1181; Krisciunas and Schaefer
1991, PASP, 103, 1033; Benn and Ellison 1998, La Palma Technical Note 115). A
model was constructed following the prescription of Krisciunas (1997) and
Krisciunas and Schaefer (1991). The graph below shows the cumulative
probability distributions of V-band sky brightness at an arbitrary phase in
the solar cycle for three model observation scenarios. In the first model
the target is always at the zenith. The second and third models are more
realistic Monte Carlo realisations of likely queue and classical programs. In
the second model targets were chosen with a gaussian distribution in Hour
Angle with sigma=1 hour, and with a distribution in Declination between -20
and +90 degrees based on the surface area of the celestial sphere. The third
model is the same as the second, but with the further constraint that the
target must be at least 30 degrees from the moon. It may seem surprising that
the results of these 3 models differ so little. This is due to the fact that
the primary dependence of night sky brightness is on lunar phase, and
secondarily on moon - target distance.

The results of these calculations indicate that the sky at Mauna Kea is
fainter than 20.78 mag/arcsec2 for 50% of the time and fainter than
21.37 mag/arcsec2 for 20% of the time for any random target. (For
an unbiased distribution of queue-mode nights the moon is below the horizon
for about half the time, of course).

The values presented in the observing
constraints table, and used in the integration time calculator, are valid
for semesters 2000B - 2001B and have been adjusted from the graph above to
those expected for a nominal solar cycle variation (currently close to the
mid-point). These values will be modified slightly each year or so.

The colour of the sky changes with lunar
phase. Adopted values are shown in the table below (taken from ESO, by scaling
V for inferred equivalent lunar phase, and from Walker, NOAO Newsletter No.
10). The conversion between the sky background category and the number of
nights from new moon indicates the constraints that are applied to schedule
classical observations and do not necessarily correspond to
conventional definitions of dark, grey and bright time.

Data from HM Nautical Almanac Office, showing sun and moon rising and setting times and
lunar phases for Mauna Kea and Cerro Pachon/Tololo/La Silla can be found here.

The broad-band sky brightnesses given in
the table above have been used to scaled a model optical sky spectrum .
These spectra are used in the Integration Time Calculators. The sky spectrum is
patched to the near-IR sky spectrum at a wavelength of 920nm. An example is
shown below (for 50%, 'dark' conditions) and the data file is available.